Projection displays are used for a wide variety of applications, such as producing the pictures viewed on television screens. A typical projection display system includes a number of components, including a display screen, a light source, and an optical path between them. To create the pictures, one or more light sources are provided to emit light when it is needed. The light they produce is then manipulated by a series of optical devices in order to create the visual image. The visual image created along the optical path is then displayed on the display screen, the television screen for example, or another visual display. In most cases, the goal is to produce the best picture possible. The key to producing a desirable visual display, of course, is the configuration of the various optical devices along the optical path. The selection, operation, and configuration of these devices also contribute to unseen characteristics of the system, such as cost and efficient use of system resources.
Several types of projection displays have recently been developed. These new display systems are now becoming much more common, serving as a replacement for the widely-used CRT (cathode ray tube) display, which produces a visual image by producing and directing a stream of electrons at a treated display surface. The stream could only be directed to one point at any given time, but can be systematically swept across the display with such speed as to create the visual impression of a single image. This technology is fairly well-developed, but has reached the point where perceptible increases in quality are difficult to achieve. A CRT also takes up a relatively-large amount of space because the components used for generating the electron stream must be placed at a certain distance from the display screen. Many recently-developed projection display systems, in contrast, feature a much slimmer profile. In addition, projection display systems often produce much cleaner visual images. The combination of these advantages has made such systems immensely popular.
One such projection-display system is commercially available from Texas Instruments of Dallas, Tex. under the trademark DLP® (or Digital Light Processing®). DLP® projection-display systems utilize a digital micromirror device (DMD) in their optical path. The DMD typically includes an array of thousands of tiny mirrors that are used to manipulate light originating at an internal light source. Lenses and other components in the optical path adjust the light for use by the DMD, or convey the image it generates to a display plane.
Most such systems utilize a total internal reflection (TIR) prism arrangement or reverse TIR(RTIR) prism arrangement. With TIR prism arrangements wherein light is internally reflected on the image side, light modulated from the DMD intersects two complementary prisms equivalent to a parallel plate. With RTIR arrangements wherein the light is internally reflected on the projection side, the prisms must be such that lateral color and geometrical aberrations introduced by both prisms are partially compensating each other. In both of these types of systems, two prisms are used to compensate for aberrations, such as lateral color, anamorphic magnification, astigmatism, and the like, that using a single prism may cause.
The use of TIR or RTIR arrangements, however, requires extremely constraining manufacturing tolerances and is expensive to fabricate and to assemble. In particular, the prisms of TIR or RTIR arrangements are generally aligned such that the surfaces of the prisms are parallel and extremely close. Furthermore, the TIR or RTIR arrangements may exhibit contrast problems due to the high angle of incidence on both surfaces creating the gap between the two prisms inducing multiple parasitic reflections close to the focal plane and therefore degrading the contrast.